Delignification of Aspen Wood Using
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In: TAPPI Notes--1989Contaminant. problems and strategies in wastepaper recycling seminar, 1989 April 24-26; Madison, WI. Atlanta, GA: TAPPI Press; 1989: 125-128. DELIGNIFICATION OF ASPEN WOOD USING HYDROGEN Acidic hydrogen peroxide is a much stronger oxidizing PEROXIDE AND PEROXYMONOSULFATE agent than alkaline hydrogen peroxide. Under acidic conditions, the lignin macromolecule is extensively Edward L. Springer, Chemical Engineer 1 degraded and dissolved by hydrogen peroxide (4,5). It USDA Forest Service, Forest Products Laboratory seemed likely, therefore, that acidic solutions of Madison, WI hydrogen peroxide should more readily delignify wood U.S.A. than alkaline solutions. ABSTRACT DELIGNIFICATION USING HYDROGEN PEROXIDE One-gram samples of aspen wood, ground to pass through Treatment of finely divided aspen wood with a No. 40 mesh screen, were treated at room temperature peroxymonosulfate at low pH, followed by alkaline (22°C) with a 5.0-percent solution of hydrogen extraction, resulted in nearly complete lignin peroxide adjusted to either pH 2.0 with sulfuric acid removal. Treatment of the wood with hydrogen peroxide or to pH 11.0 or 11.5 with sodium hydroxide. A liquor at optimum pH (pH 11). followed by alkaline to wood ratio of 10:1 was used, and the treatment time extraction, removed at most 36 percent of the original was 3 days except for the pH 11.5 condition where lignin. Peroxymonosulfate can be easily produced by treatment time was 12 days. Samples were also treated mixing hydrogen peroxide with concentrated sulfuric at the natural pH (pH 5.5) of the 5.0-percent solution acid. of hydrogen peroxide. After peroxide treatment, each sample was extracted with 1.0-percent sodium hydroxide Keywords: Delignification, hydrogen peroxide, solution at 50°C for 3 h. The results from these sulfuric acid, peroxymonosulfuric acid, potassium treatments are shown in Table I. peroxymonosulfate, oxone, aspen wood, Populus tremuloides Michx., sodium hydroxide The most selective delignification occurred at pH 11.0 where 32 percent of the original lignin and 17 percent of the original carbohydrate were removed. Dividing INTRODUCTION the total weight of carbohydrate removed by the weight of lignin removed (C/L ratio) gives an index of the Wood and other lignocelluloses can be readily selectivity of lignin removal (Table I). Increasing delignified using organic peroxides such as peracetic the pH to 11.5 and the treatment time to 12 days and performic acids (1). With the exception of resulted in a small increase in lignin removal (to studies using alkaline hydrogen peroxide, little 36 percent of the original); however, the selectivity research has been done on delignification with was slightly reduced (from C/L ratio 2.2 to 2.4). inorganic peroxides. The purpose of this work was to Lowering the pH to 2.0 increased the lignin removal determine whether acidic hydrogen peroxide could only slightly from what it was at pH 5.5. Much more delignify aspen wood and, if not, whether lignin was removed under alkaline conditions. peroxymonosulfuric acid made by reacting hydrogen peroxide and sulfuric acid could delignify the wood. To determine the effect of treatment temperature on Aspen wood (Populus tremuloides Michx.) was chosen for lignin removal, runs were made at 80°C. The results study because it is easily delignified. from these runs are also shown in Table I. The most effective delignification again occurred at pH 11.0; A few studies have been performed on delignification however, the selectivity of lignin removal was of lignocellulosic materials with alkaline hydrogen somewhat reduced from that at room temperature. None peroxide. Gould (2) found that approximately half the of the conditions studied resulted in low lignin lignin present in agricultural residues, such as wheat contents in the final residue. Since acidic hydrogen straw, could be solubilized when the residue was peroxide was ineffective in delignification of aspen treated at 25°C with an alkaline solution of wood, we decided to attempt to delignify the wood with hydrogen peroxide. The delignification was most peroxymonosulfuric acid produced by adding sulfuric effective at pH 11.5. McDonough et al. (3) studied acid to hydrogen peroxide. the delignification of Southern Pine kraft pulp with alkaline hydrogen peroxide. They also found that GENERATION OF PEROXYMONOSULFATE approximately half the lignin present in the pulp could be removed. In both cases, the most efficient Adding concentrated sulfuric acid to solutions of delignification occurred under reaction conditions hydrogen peroxide results in the production of where no stabilizers (to inhibit peroxide peroxymonosulfuric acid (6). This acid is a much decomposition) were present and the rate of peroxide stronger oxidizing agent than hydrogen peroxide. It decomposition was at a maximum. For comparison, we can also be produced by adding peroxydisulfuric acid initially studied the delignification of aspen wood salts to concentrated sulfuric acid. Zakis and with alkaline hydrogen peroxide. Neiberte (7) found that they could delignify spruce sawdust to low lignin levels using peroxymonosulfuric ----------------------------- acid in 50-percent sulfuric acid at 20°C. They produced the peroxymonosulfuric acid by adding the ammonium salt of peroxydisulfuric acid to concentrated 1The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This sulfuric acid. A high concentration of sulfuric acid article was written and prepared by U.S. Government seemed to be necessary for effective lignin removal. employees on official time, and it is therefore in the Based on these findings, hydrogen peroxide and public domain and not subject to copyright. sulfuric acid mixtures with a large excess of acid present should delignify finely divided aspen wood. 1989 Contaminant Problems and Strategies in Wastepaper Recycling / 125 Our previous data indicate that simply adjusting a Comparing runs 6 and 4, the same equivalent 5.0-percent solution of hydrogen peroxide to pH 2.0 concentration of hydrogen peroxide is present; with concentrated sulfuric acid did not result in a however, the initial concentration of solution that effectively delignified aspen wood. peroxymonosulfate was greatly increased in run 6. The Several experiments were performed to determine the treatment time for run 6 was reduced to 1 day. conditions needed to produce significant quantities of Despite this reduced treatment time, run 6 shows a peroxymonosulfuric acid. Cold hydrogen peroxide large increase in lignin removal compared with run 4. (2°C) was placed in a vial in an ice bath: room This is undoubtedly an effect of the much higher temperature concentrated sulfuric acid was then added, initial concentration of peroxymonosulfate. and the two were thoroughly mixed. An aliquot of this Another source of the peroxymonosulfate anion is mixture was titrated using the method of Greenspan and Oxone, 2 a commercial product sold by DuPont. 3 MacKellar (8) to determine the yield of peroxymonosulfuric acid produced. Results of these Oxone is a triple salt containing potassium peroxymonosulfate (2 KHSO KHS · KHSO · procedures are shown in Table 11. Using 32-percent 5 4 hydrogen peroxide and a mole to mole ratio of acid to K SO ). Delignification of aspen wood with Oxone was peroxide of 1:1, the yield of peroxymonosulfate was 2 4 only 8 percent. This indicates that the yield was compared with delignification using peroxymonosulfate probably negligible when a few drops of concentrated from the peroxide-acid mixtures. In run 7, an sulfuric acid were added to 5.0-percent hydrogen equivalent quantity of Oxone was substituted for the peroxide to adjust the pH to 2.0. Using more procedure 3 mixture in run 6. All other conditions concentrated hydrogen peroxide and a higher mole to were identical except that some hydrogen peroxide was mole ratio of acid to peroxide greatly increased the present in run 6 and the pH was much lower. The yield of peroxymonosulfate. results were quite similar. For run 7, the yield of residue was a bit higher and its lignin content also DELIGNIFICATION USING PEROXYMONOSULFATE somewhat higher. These significant differences were probably due to the much lower pH in run 6. This is Small quantities of the solutions containing reflected in the large difference in residue peroxymonosulfate were carefully diluted with viscosities in runs 6 and 7. The much higher hydrogen distilled water and used to treat one gram samples of ion concentration in run 6 undoubtedly increased the the No. 40 mesh aspen wood at room temperature rate of carbohydrate degradation and also increased (22°C). The weight of the peroxide-acid mixture the rate of lignin removal. If this lower pH were used for a given total weight of solution (used to taken into account, it appears that, with regard to treat 1.00 g of wood) is shown in the third column of delignification, the source of the peroxymonosulfate Table 111. The concentration of the peroxymonosulfate anion is of no importance. anion in the initial solution is given in column 4, and the equivalent total percentage of hydrogen CONCLUSIONS AND RECOMMENDATIONS peroxide in the initial solution is given in column 8. The treated residues were extracted as described Low pH solutions of the peroxymonosulfate anion are previously with 1.0-percent sodium hydroxide. much more effective in delignifying aspen wood than are alkaline solutions of hydrogen peroxide. This is Initial runs were made using peroxymonosulfate probably because, under these conditions, prepared by procedure 1. This resulted in very dilute peroxymonosulfate is a much stronger oxidizing agent solutions of peroxymonosulfate containing large than is hydrogen peroxide. The very low pH of the amounts of acid and peroxide. In the first run peroxymonosulfate solutions, produced by mixing (Table III), the initial concentration of hydrogen peroxide with sulfuric acid, results in peroxymonosulfate was only 0.45 percent and the marked attack on the carbohydrate constituents of the equivalent total concentration of hydrogen peroxide wood, resulting in low residue yields and low was 2.3 percent.